I set a Data Validation (Tools -> Data Validation) and setup something that will never be entered. Something like a number between -918234 and -918234 might do you. Be sure to remove all Help text and set "Allow cells to contain only valid data". You can still enter a formula into the cell but no text/numbers.

Please don't even call this 'pseudo-locking'! If the Google Docs spreadsheet team thinks this may work as a 'locking' mechanism for cells they may never implement real locking. (Also note Data Validation is incompatible with the List View feture recently introduced and with forms).

OMG,You mean I spent 4 hours converting a spreadsheet from Excel and they haven't even implemented cell locking ?Surely this isn't still a beta product....I guess I was expecting too much, such an important feature when collaborating and creating foolproof spreadsheets for others to use.....Any Ideas hen it might happen ?

They just implemented a feature to lock entire sheets, but not individual cells. This would be a pretty important addition which may actually scale into a feature that prescribes cell security rules. As in, you can dictate that only the spreadsheet owner has access to change those cells --- or --- actually dictate which shared users are allowed to change those cells.

I think this is very important! I made a sheet where users can enter data and learning the price of the product they are intrested in. But I realized that they don't enter their data only in the "yellow" cells, as I warned, so they destroy all the formulas.... This "lock cell" tool, is to necessary.

I really need this feature. I keep my budget in a Google spreadsheet. Some of the cell values are calculated, such as how much interest and principle I pay on a loan each month. I want to protect these cells to protect the formulas, which can be quite complex. For example:

I need the feature very badly. I share lot of spreadsheets with other users online who are supposed to modify only one column data but many times they accidentally delete data from other columns. Let's hope it is implemented soon.

I need this too. I`musing it so that people can find the price for a online calculator for sliding systems . People should be able to edit only where i allow, or the next user that will come and try to find the price, will find out that it`s another price...

Yes, I need this. Also, it would be great if cells, columns, and rows could be locked for a set of users that I share it with, but not for others or myself. I would like to be able to have some users be able to edit portions of my spreadsheets, and others to be able to edit all portions.